It is known that elastic substances, such as, e.g., a silicone gel, possess excellent softness and stress-relaxation properties. However, a problem experienced with their use is associated either with low shape-preserving properties or low strength, or with tackiness that makes them difficult to handle in production. It has been proposed to solve the above problems by forming a layered silicone composite material consisting of a soft elastic underlayer and a relatively hard elastic material on the top of the soft layer. For example, Japanese Unexamined Patent Application Publication (hereinafter referred to as Kokai) 562-148901 and Kokai S62-239106 (equivalent to DE3712145) disclose an optical material in the form of a layered composite structure consisting of a plurality of elastic layers having different hardness. Kokai 2000-150968 (equivalent to U.S. Pat. No. 6,590,235) describes a layered silicone composite material for use as a cover for optical elements such as LEDs, wherein the aforementioned material also consists of a relatively soft inner silicone layer and a relatively hard outer silicone layer. However, when a layered silicone composite material is made from a curable silicone composition, and in particular, from a silicone composition curable via an addition reaction (hereinafter referred to as an addition-curable silicone composition), it becomes difficult to firmly interconnect the layers into a single body in view of the poor adhesion between the layers.
An attempt to solve the above problem is described in Kokai H5-69511 and Kokai H5-69512, where a method is proposed for forming a layered silicone composite material produced from a silicone gel substrate layer and a silicone elastomer or silicone resin surface layer by simultaneously curing curable silicone compositions of different specific gravity and miscibility. A drawback of the aforementioned method is that it is difficult to control the thickness and hardness of individual layers and that the interface between the layers has a non-uniform nature.
Kokai H1-34370 (equivalent to EP0300620) discloses a method, in which a layered structure is formed by interconnecting through adhesion a silicone elastomer layer and a silicone gel layers which are both in a cured state. Such a method, however, results in poor adhesion between the layers, as well as between the obtained layered structure and a substrate, if such a connection is required.